1. Field of the Invention
The present invention relates to a memory access control apparatus suitable for a personal computer.
2. Description of the Related Art
In a conventional computer system having a relatively small capacity such as a personal computer, when a large-capacity file is to be accessed at high speed, a hard disk unit is generally used. However, when a hard disk unit is used in a portable personal computer driven by a battery, the following problems are posed:
(i) Since a motor must be constantly rotated in the hard disk unit, a current consumption is large, and the service life of the battery is short.
(ii) In order to eliminate the problem of item (i), the motor may be automatically stopped when the hard disk is not used and be restarted when it is used. In this case, however, an access time is prolonged, or the service life of the hard disk is shortened because the frequency of vertical movement of the head is increased.
(iii) The portable personal computer is more likely subjected to the influence of vibrations or a shock and the hard disk is more likely to crash.
For these reasons, an architecture called a RAM disk or a virtual disk (to be simply referred to as a RAM disk hereinafter) has been developed. In this system, a data access is made by using a physical memory address space as a disk unit. This system allows processing at a speed higher than in a floppy disk unit or a hard disk unit by utilizing the characteristics of an IC memory. FIG. 1 shows the principle of a RAM disk system from the viewpoint of software. Application program 1 transfers control to the MS-DOS 3 through a DOS interface program such as the MS-DOS (disk operation system) interface program available from Microsoft Corporation. If a control command from application program 1 is supplied to floppy disk drive (FDD) 7 or hard disk drive (HDD) 9, MS-DOS 3 transfers control to ROM-BIOS (basic input/output system) 5 by executing interrupt instruction INT 13H (e.g., a software interrupt in a 16-bit microprocessor 8086 system available from Intel Corporation) of the ROM-BIOS interface program. ROM-BIOS 5 performs input/output control with respect to FDD 7 or HDD 9. When a RAM disk in main memory 13 is to be accessed, DOS 3 transfers control to RAM disk driver 11. RAM disk driver 11 accesses main memory 13.
However, according to the conventional system shown in FIG. 1, since RAM disk driver 11 is incorporated in MS-DOS 3 as an optional driver, the following drawbacks are posed:
(i) It is compatible with a normal disk unit through a DOS interface, but not compatible through ROM-BIOS 5. For this reason, some programs cannot access the RAM disk in the same manner as in a normal disk unit.
(ii) Since a memory access means is incorporated in the system as an optional driver of the MS-DOS, the RAM disk area is initialized when the DOS is started.
In addition, if a personal computer is equipped with only a single floppy disk, a DOS disk must be inserted into a floppy disk drive every time the DOS is started, or control is transferred from the application program to DOS.